<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <input type="text" placeholder="邮箱/ID/手机号" id="">
</body>
<script>
    var ipt=document.querySelector('input')
    ipt.onfocus=function(){
        ipt.placeholder='';  
        
    }
    ipt.onblur=function(){
        ipt.placeholder='邮箱/ID/手机号';  

    }

</script>

</html>